The main purpose of metal detectors of the kind described herein is to detect the presence of metal in an article, a bulk material, or generally any object being examined. Such metal detectors are widely used and integrated into production and packaging lines, for example to detect contamination of food by metal particles or components from broken processing machinery during the manufacturing process, which constitutes a major safety issue in the food industry. The generic type of metal detector that this invention relates to and which is known as balanced three-coil system with an encircling coil arrangement can be described as a portal through which the articles and materials under inspection are moving, for example individual packages riding on a horizontal conveyor belt through a vertical portal, or a stream of bulk material in free fall through a vertical duct or funnel passing through a horizontally arranged portal.
The portal is generally configured as a box-shaped metallic enclosure with an entrance aperture and an exit aperture. The operative part of the metal detector is a system of three electrical coils wound on a common hollow carrier or coil former made of a non-metallic material, which is arranged inside the metallic enclosure. The aperture cross-section of the coil former matches the size and shape of the entrance and exit apertures and lines up with them, so that the coil former and the entrance and exit apertures form a tunnel defining a detection zone through which the conveyor belt or other transport means moves the articles or materials under inspection. The cross-section of this detection zone tunnel is generally rectangular or circular, but could also have any other shape.
In state-of-the-art metal detectors of this type, the coils are exactly parallel to each other and, consequently, their parallel planes are orthogonal to their common central axis. The center coil, also variously called transmitter coil, emitter coil, or excitation coil, is connected to a high-frequency oscillator and thus generates a primary alternating electromagnetic field which, in turn, induces a first and a second alternating voltage, respectively, in the two coils on either side of the center coil, which are also called the first and the second receiver coil. The first and second receiver coils are connected in series with each other, but with their windings wired in opposition to each other. In other words, the coil wire runs continuously from a first output terminal through the windings of the first receiver coil, then with the opposite sense of rotary direction through the windings of the second receiver coil to a second output terminal. In addition the first and second receiver coils are located equidistant from the transmitter coil. Therefore, they are in all respects mirror images of each other in relation to the central plane of the transmitter coil, and thus the first and the second alternating voltage induced in them by the primary alternating electromagnetic field will cancel each other. In other words, the mirror symmetry of this state-of-the-art metal detector has the result that the voltage picked up between the first and second output terminals will be zero.
Symmetrical balance coil arrangements can also consist of multiple transmitter coils and/or multiple receiver coils that are arranged in such a way to achieve a so called null balance condition. Therefore the first receiver coil can form one or more entrance-side receiver coils, and the second receiver coil one or more exit-side receiver coils. Likewise the transmitter coil can be designed as one or more transmitter coils.
However, if a piece of metal passes through the coil arrangement, the electromagnetic field is disturbed, giving rise to a dynamic voltage signal across the output terminals of the serially connected receiver coils.
The foregoing concept, often referred to as “balanced-coil system”, “inductively balanced metal detector” and similar terms, is commonly known in the field of industrial metal detectors. The generic principle is described and illustrated for example in U.S. Pat. No. 4,563,645 (col. 1, lines 11-32, and FIG. 1) as well as U.S. Pat. No. 7,061,236 B2 (col. 1, lines 20-41, and FIGS. 1 and 2a).
The metallic enclosure surrounding the coil arrangement serves to prevent airborne electrical signals or nearby metallic items and machinery from interfering with the proper functioning of the metal detector. In addition, the metal enclosure adds strength and rigidity to the assembly, which is absolutely essential as even microscopic dislocations of the coils relative to each other and relative to the enclosure can disturb the detection system which is sensitive to signals in the nanovolt range.
An issue of concern in metal detectors of the foregoing description is their sensitivity to stationary and, even more so, to moving metal in areas outside the detection zone and, in particular, even far outside the enclosure of the metal detector. This is due to the fact that the electromagnetic field generated by the transmitter coil extends outside the entrance and exit apertures to a distance as far as two or three times the length of the detection zone. If there are stationary or moving metal parts within this range, for example the support frame or other components of a conveyor, the interaction of the electromagnetic field with the metallic parts in its reach will produce an unwanted output signal of the receiver coils which interferes with the actual detection signals originating from metallic contaminants in the material under inspection traveling through the metal detector. Therefore, unless special design measures are taken, a large space before the entrance aperture and after the exit aperture of the metal detector has to be kept free of all metal. The area that must be kept free of metal in order to ensure the proper operation of a balanced-coil metal detector is generally called the “metal-free zone” or MFZ.
The metal-free zone, in particular its length in the direction of the transport path, is normally specified as a multiple of the aperture height (or diameter) h for stationary metal and for moving metal. According to EP 0 536 288 B1 (col. 2, lines 6-8, and FIG. 1), the MFZ extends to about 1.5×h for stationary metal and to about 2×h for moving metal. In any given application, the MFZ will dictate the metal detector system design, specifically the insertion space, i.e. the amount of space that must be allowed in a packaging or process line to accommodate the metal detector and its MFZ.
In applications where the space available for the metal detector is limited and where the foregoing guideline can therefore not be met, the interference due to metallic objects in the ambient vicinity could be suppressed by lowering the sensitivity of the metal detector to the point where the spurious signals are no longer registered. Of course, this would simultaneously reduce the useful detection sensitivity for metal contaminants inside the detection zone, i.e. it would handicap the metal detector in a clearly undesirable way.
A solution whereby the metal-free zone in the type of metal detector described hereinabove is reduced or even eliminated is presented in EP 0 536 288 B1, which is hereby incorporated by reference in the present disclosure. One of the possible means for reducing or eliminating the MFZ described in EP 0 536 288 B1 has the form of metallic flanges or collars that may be integral with the rims of the entrance and exit apertures of the enclosure of the metal detector. These flanges or collars act as short-circuit coils in which a current is induced by the alternating electromagnetic field of the transmitter coil. The induced current, in turn, generates a secondary electromagnetic field which can, under certain conditions, nullify the primary field of the transmitter coil beyond a certain distance before the entrance coil and after the exit coil, even to the extent that the primary field outside the apertures of the enclosure is totally suppressed and the metal-free zones before the entrance aperture and after the exit aperture are effectively reduced to zero providing a so-called “zero metal-free zone” (ZMFZ).
Because a state-of-the-art metal detector using the ZMFZ concept according to EP 0 536 288 B1 can be operated at the full detection sensitivity that it was designed for, even with metallic structures or machinery adjacent to one or both of its apertures, it is advantageous for installations where there is not enough space available to allow for the metal-free zones that would be required with a metal detector of an earlier state of the art. Nevertheless, the full sensitivity of a metal detector equipped with aperture flanges according to the ZMFZ concept is lower than the full sensitivity of a conventional metal detector operating with the required metal-free zones. Thus, the present state of the art still represents a compromise: while a ZMFZ metal detector significantly reduces or even eliminates the need for metal-free zones upstream and/or downstream in the processing line, it comes at the expense of a somewhat lower detection sensitivity in comparison to a conventional metal detector installed in a longer insertion space that allows for the metal-free zones.